JInternalframe resizing

I have an application which consists of a JFrame containing an internal menu in the form of a JInternalframe + JTree. When the form opens by default it fills the screen and the JInternalframe containing the menu occupies the left hand side of the screen. I would like to capture any events which result from the user re-sizing the main window so that the internal frame is resized to fit. Currently the bottom of the internal frame just disappears out of view if the user reduces the height of the main window.

ok you've sorted that for me. Is there something similar that I should do to handle the re-positioning of all my minimised jinternalframes once the containing jframe is resized. Currently they disappear out of sight if the frame is reduced.

I'm sure it's simply, just can't think of the right phase to search for examples.